Procedures and devices for operating a combustion engine are known, in whose exhaust gas area a SCR-catalyzer (selective catalytic reduction) is arranged, which reduces the nitrous gases (NOx) that are contains in the exhaust gas of the combustion engine into nitrogen in the presence of a reducing agent. Thereby the nitrous gases in the exhaust gas can be significantly reduced. Preferably nitrous gases (NO, NO2) are hereby reduced, while undesired side-reactions are mostly suppressed. Ammonia (NH3) is required for the course of the reaction, which is mixed to the exhaust gas. Therefore NH3 or NH3-splitting reagents are used as reactant. Usually a watery urea solution is therefore used, which is injected before the SCR-catalyzer into the exhaust gas system with the aid of a metering device. From the solution NH3 is created, which works a reducing agent.
The metering of the reactant preferably takes place depending on the power-operated nitrous gas emissions and is therefore depending on the momentary engine speed and the torque of the engine. The metering is therefore carried out depending on operating parameters of the combustion engine and depending on exhaust gas parameters.
The metering of the reactant has to be determined carefully. At a too low metering the nitrous gases in the SCR-catalyzer cannot be reduced completely anymore. At a too high metering a so-called reactant slip (NH3-slip) can occur, which causes on the one hand an odor nuisance by the released ammonia and on the other hand an unnecessarily high consumption of the reactant.
The efficiency of a SCR-catalyzer depends on the temperature and also significantly on the NH3-filling level. SCR-catalyzers accumulate by absorption a certain amount of ammonia on their surface. Thereby also stored NH3 is available for the nitrous gas reduction besides the directly metered ammonia in the form of an urea water solution, whereby the efficiency towards an empty catalyzer increases. The storage behavior depends on the operating temperature of the catalyzer, which means the lower the temperature the higher the storage capacity.
If the catalyzer has filled its storage completely a NH3 slip can occur at load jumps. This can be the case even if no reactant is injected anymore. Because it is usually desired to achieve a high nitrous gas conversion it is required to operate the SCR-catalyzer at a high NH3-filling level. Therefore a NH3-slip can shortly occur even at an exactly planned metering amount under unsteady conditions.
In order to optimize the metering of the reactant DE 10 2004 031 624 A1 suggests a procedure for operating a SCR-catalyzer that is used for purifying the exhaust gas of a combustion engine, at which a controlling or a regulation of the filling level of the reactant, in particular the NH3-filling level, is provided in the SCR-catalyzer on to a default threshold value. The planned default of the threshold value shall allow that in particular at unsteady states of the combustion engine on the one hand a sufficient amount of reactant is provided for the catalytic reduction of nitrous gases and that on the other and a slip of the reactant, in particular of NH3, is avoided. The reactant filling level of the SCR-catalyzer is determined with the aid of a catalyzer model (SCR-model). Thereby the NOx-mass flow that is flowing into the SCR-catalyzer, the NOx-mass flow that is leaving the SCR-catalyzer, the catalyzer temperature as well as the reactant slip are considered. The efficiency of the SCR-catalyzer depends on the catalytic activity, which is low at low operating parameters, passes a maximum with an increasing operating temperature and sinks again at a more increasing operating temperature. Even the maximally possible reactant filling level of the SCR-catalyzer depends on the operating temperature of the SCR-catalyzer as described above.
The calculation of the necessary amount of the reactant is subject to numerous errors and deviations, for example the engine raw emissions, the conversion rate of the catalyzer and also inaccuracies of the metering system have an influence on the calculation of the metering amount. Therefore an adaptation of the filling level is required. In order to carry out an adaptation a NOx-sensor is usually used, which can detect the nitrous gas amount downstream of the SCR-catalyzer. Due to the measuring principle at known NOx-sensors these sensors show a lateral sensitivity towards NH3. Therefore a usually used NOx-sensor measures a sum signal of NOx and NH3. The SCR-model calculates on the other hand only the NOx-emissions after the SCR-catalyzer. Therefore deviations from the measured NOx-sensor value, a so-called slip, can have three causes: Besides the model inaccuracy as first cause an underestimation of the filling level and therefore a release of NH3 (NH3-slip) and an overestimation of the filling level and therefore a minimal conversion and release of NOx (NOx-slip) can be the cause of the deviation.
In order to carry out an adaptation of the metering system first an increase of the calculated actual filling level in the SCR-catalyzer is usually carried out at a deviation of the sensor value from the calculated value. As a reaction upon this the metering amount of the reactant is taken back by the system. But because the NH3-slip causes a deviation between the calculated and the measured value in the same direction at too high NH3-filling level and also at a too low nitrous gas at a too low NH3-filling level, it cannot be distinguished between NH3-slip and too low conversion (NOx-slip). Therefore the adaptation reacts in the wrong direction, as long as a too low nitrous gas conversion (minimal conversion) is present. The nitrous gas conversion diminishes hereby more. It is therefore required to carry out a reasonability test of the adaptation, in order to detect an erroneous adaptation and to correct it if necessary.
In the course of the reasonability check it is decided whether the intervention into the metering of the reactant by increasing the actual filling level in the SCR-model was correct or not. Hereby the calculated conversion is compared with the metered reactant +NH3 from the desorption as well as the change of the NH3-nominal filling level. If these relations are not reasonable an incorrect adaptation is detected, which means the cause for the deviation of the calculated NOx-value and measured NOx-value was based on an under-metering or a minimal conversion and not on a NH3-slip. Correspondingly the metering strategy is newly installed. In order for the SCR-system to adjust again to the actual operating point the filling level adaptation as well as reasonability test are locked for an applied time.
This procedure has several disadvantages. Thus the storage filling level in the SCR-catalyzer is increased at a detected NH3-slip at regular intervals; as a result the metering amount is reduced, so that conversion drops can occur in the nitrous gas conversion with consequences for the exhaust gas composition during the adaptation. Furthermore long reasonability check phases, for example phases of 200 seconds or more, are necessary for the comparison between the relation of calculated conversion and metered reactant +NH3 from the desorption as well as the change of the NH3-nominal filling level. Therefore the period of time until the optimal adaptation of the system is relatively long and until then a not optimal function in the SCR-catalyzer exists. In addition the reasonability check result has often to be applied in an uncertain manner and the function difficulty.
The invention has therefore the task to avoid the mentioned disadvantages and to provide a new reasonability check procedure at the operation of a combustion engine with a SCR-catalyzer, which enables a reliable result of the reasonability check and which simultaneously allows very fast statements about the reasonability.
This task is solved by a procedure for operating a combustion engine, as it is described in claim 1. Preferred embodiments of this procedure are stated in the sub-claims.